We present results from a detailed experimental investigation of LaFeAsO, the parent material in the series of "FeAs" based oxypnictide superconductors. Upon cooling, this material undergoes a tetragonalorthorhombic crystallographic phase transition at ϳ160 K followed closely by an antiferromagnetic ordering near 145 K. Analysis of these phase transitions using temperature dependent powder x-ray and neutrondiffraction measurements is presented. A magnetic moment of ϳ0.35 B per iron is derived from Mössbauer spectra in the low-temperature phase. Evidence of the structural transition is observed at temperatures well above the transition temperature ͑up to near 200 K͒ in the diffraction data as well as the polycrystalline elastic moduli probed by resonant ultrasound spectroscopy measurements. The effects of the two phase transitions on the transport properties ͑resistivity, thermal conductivity, Seebeck coefficient, and Hall coefficient͒, heat capacity, and magnetization of LaFeAsO are also reported, including a dramatic increase in the magnitude of the Hall coefficient below 160 K. The results suggest that the structural distortion leads to a localization of carriers on Fe, producing small local magnetic moments which subsequently order antiferromagnetically upon further cooling. Evidence of strong electron-phonon interactions in the high-temperature tetragonal phase is also observed.

Measuring temperature and heat flux is important for regulating any physical, chemical, and biological processes. Traditional thermopiles can provide accurate and stable temperature reading but they are based on brittle inorganic materials with low Seebeck coefficient, and are difficult to manufacture over large areas. Recently, polymer electrolytes have been proposed for thermoelectric applications because of their giant ionic Seebeck coefficient, high flexibility and ease of manufacturing. However, the materials reported to date have positive Seebeck coefficients, hampering the design of ultra-sensitive ionic thermopiles. Here we report an “ambipolar” ionic polymer gel with giant negative ionic Seebeck coefficient. The latter can be tuned from negative to positive by adjusting the gel composition. We show that the ion-polymer matrix interaction is crucial to control the sign and magnitude of the ionic Seebeck coefficient. The ambipolar gel can be easily screen printed, enabling large-area device manufacturing at low cost.

The thermal conductivity kappa of the layered s-wave superconductor NbSe2 was measured down to T(c)/100 throughout the vortex state. With increasing field, we identify two regimes: one with localized states at fields very near H(c1) and one with highly delocalized quasiparticle excitations at higher fields. The two associated length scales are naturally explained as multiband superconductivity, with distinct small and large superconducting gaps on different sheets of the Fermi surface. This behavior is compared to that of the multiband superconductor MgB2 and the conventional superconductor V3Si.

A mixed ionic–electronic conductor based on nanofibrillated cellulose composited with poly(3,4‐ethylene‐dioxythio­phene):­poly(styrene‐sulfonate) along with high boiling point solvents is demonstrated in bulky electrochemical devices. The high electronic and ionic conductivities of the resulting nanopaper are exploited in devices which exhibit record values for the charge storage capacitance (1F) in supercapacitors and transconductance (1S) in electrochemical transistors.

We have synthesized epitaxial Sr 2 IrO 4 thin-films on various substrates and studied their electronic structures as a function of lattice-strain. Under tensile (compressive) strain, increased (decreased) Ir-O-Ir bond-angle is expected to result in increased (decreased) electronic bandwidth. However, we have observed that the two optical absorption peaks near 0.5 eV and 1.0 eV are shifted to higher (lower) energies under tensile (compressive) strain, indicating that the electronic-correlation energy is also affected by in-plane lattice-strain. The effective tuning of electronic structures under lattice-modification provides an important insight into the physics driven by the coexisting strong spin-orbit coupling and electronic correlation. PACS: 71.70.Ej, 72.80.Sk, 81.15 In this letter, we report on the growth and optical properties of Sr 2 IrO 4 (SIO-214) thin films. The in-plane lattice mismatches between SIO-214 and various oxide substrates can exert both tensile (+) and compressive (-) strains to films, as shown in Fig. 1(a). We find that the electronic structure of SIO-214 films are effectively altered by lattice strain, and we observe 3 shifted optical transitions (absorptions) between the J eff = 1/2 lower Hubbard band (LHB) and the J eff = 1/2 upper Hubbard band (UHB), and between the J eff = 3/2 band and the J eff = 1/2 UHB band. Our observations strongly suggest that not only the electronic bandwidth, but also the magnitude of the effective electronic correlation energy (U eff ), can be manipulated by lattice strain. Our results demonstrate that epitaxial SIO-214 thin films can be used as a model system to study the physics of coexisting strong electron correlation and strong spin-orbit coupling under lattice modification.We have used a custom-built, pulsed laser deposition system equipped with in-situ Table I. The epitaxial growth conditions are found to be the following: an oxygen partial pressure (P O2 ) of 10 mTorr, a substrate temperature of 700 °C, and a laser (KrF excimer, λ = 248 nm) fluence of 1.2 J/cm 2 . Figure 2 shows θ-2θ X-ray diffraction scans of the samples discussed herein. Well-defined 00l-peaks are present due to the films' 00l-orientation along the perpendicular to the substrates. The full widths at half maximum in rocking-curve scans of the 00l peaks are all less than 0.05°, which confirms the high crystallinity of the films. Note that the thin films' 0012-peaks are shifted to low angles as the substrate lattice parameters decrease (from GSO to LAO). This behavior is consistent with the schematic diagrams in Fig. 1(b), since elongated (contracted) out-of-plane lattice parameters are expected as compressive (tensile) in-plane strain is exerted on thin films. 4Figure 3(a) shows X-ray reciprocal space maps, which reveal important information about both the in-plane and the out-of-plane lattice parameters of the SIO-214 thin films near the 332-reflection (103-reflection) of orthorhombic (pseudo-cubic) substrates. The 1118-peaks from the thin films are clearly observed, and are...